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CALIFA Spectroscopy of the Interacting Galaxy NGC 5394 (Arp 84): Starbursts, Enhanced [NII] 6584 and Signs of Outflows and Shocks

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CALIFA Spectroscopy of the Interacting Galaxy NGC 5394 (Arp 84): Starbursts, Enhanced [NII] 6584 and Signs of Outflows and Shocks Mon. Not. R. Astron. Soc. 000, 1{?? (2015) Printed 20 October 2018 (MN LATEX style file v2.2) CALIFA Spectroscopy of the Interacting Galaxy NGC 5394 (Arp 84): Starbursts, Enhanced [NII]6584 and Signs of Outflows and Shocks Nathan Roche1 ?, Andrew Humphrey1, Jean Michel Gomes1, Polychronis Papaderos1, Patricio Lagos1, Sebasti´anF. S´anchez2 1 Instituto de Astrof´ısica e Ci^enciasdo Espa¸co,Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal. 2Instituto de Astronom´ıa,Universidad Nacional Auton´omade M´exico, A.P. 70-264, 04510, M´exico, D.F. 23 June 2015 ABSTRACT We investigate the spiral galaxy NGC 5394, which is strongly interacting with the larger spiral NGC 5395 (the pair is Arp 84), using optical integral-field spectroscopy from the CALIFA (Calar Alto Legacy Integral Field Area) survey. Spatially-resolved equivalent widths, emission-line ratios and kinematics reveal many features related to the interaction, which has reshaped the galaxy. Hα maps (with other diagnostic emission lines) show a concentrated central (r < 1 kpc) starburst and three less luminous star-forming regions (one knot far out in the northern arm), and we estimate −1 the dust-corrected total star-formation rate as 3.39 M yr . However, much of the galaxy, especially the outer tidal arms, has a post-starburst spectrum, evidence of a more extensive episode of star-formation a few ×108 yr ago, triggered by the previous perigalacticon. The [NII]6584=Hα ratio is high in the nucleus, reaching 0.63 at the −3 centre, which we interpret as related to high electron density (ne ' 750 cm from 6717 the [SII] 6731 ratio). We find a central region of strong and blueshifted NaI(5890,5896) absorption, indicative of a starburst-driven outflow from the nucleus at an estimated velocity ∼ 223 km s−1. The CALIFA data also show an annular region at radii 2.25{4 kpc from the nucleus, with elevated ratios of [NII], [OI]6300 etc. to the Balmer lines { this is evidence of shock excitation, which might be the result of interaction-triggered gas inflow. Key words: galaxies: individual: NGC 5394, galaxies:interactions, galaxies: evolu- tion, galaxies:starburst arXiv:1507.07150v2 [astro-ph.GA] 29 Aug 2015 1 INTRODUCTION of spirals, which appear to be destined to merge. This well- known system was included in the Arp (1966) catalog of pe- Mergers of two spiral galaxies are one of the most impor- culiar galaxies under the single name of Arp 84, and studied tant processes in galaxy evolution, and typically occur in a further by Arp (1969). On account of its shape it has also sequence of stages over several hundred Myr, with successive been called the Heron Galaxy, with the larger NGC 5395 be- close passages (perigalactica) of the pair, multiple bursts of ing the `body and wings' and the two-armed, northern NGC star-formation, and complex morphological transformations 5394 the `neck, head and beak'; this is well-depicted on the preceding the final coalescence into a single galaxy (which Gran Telescopio Canarias gallery image (Fig 1). could become an elliptical or lenticular). Interacting galax- NGC 5394, a barred spiral seen almost face-on and ies at any stage can be well studied by integral field spec- classed as SB(s)b-pec, appears relatively symmetric, but its troscopy (IFS), which may reveal both the current activity structure is unusual and greatly modified by the interaction. and recent history within each resolution element. Within its compact disk it has a small, bright nucleus and 3 NGC 5394 and NGC 5395 are an interacting close pair arms (double on the west) and beyond the disk (radii r > 13 arcsec out to ∼ 40 arcsec) a distinct and much larger system of two long arms extending north and south, the southern ? [email protected] arm reaching NGC 5395. c 2015 RAS 2 N.D. Roche, A. Humphrey, J.M. Gomes, P. Papaderos, P. Lagos, S.F. S´anchez The Arp 84 system is described in some detail by Kauf- man et al. (1999), hereafter K99, who observed in Hα with a Fabry-Perot. NGC 5394 and 5395 have a mass and luminos- ity ratio about 1:4, projected separation 28 kpc, and rotate anticlockwise and clockwise respectively, so that their arms are trailing. The interaction (an orbit approximately in the sky plane) is prograde with respect to the rotation of NGC 5394, but retrograde and inclined for NGC 5395. NGC 5394 has strong Hα emission (indicating star-formation) from the nucleus (2:2×10−13 erg cm−2s−1 from Keel et al. 1985) and more from a region in the SW of the disk (the inner western arm) and two bright spots on the inner and outer north- ern arm. K99's radio observations give the neutral hydrogen 8 (HI) mass in NGC 5394 as 7:3 × 10 M , and the CO data (tracing molecular hydrogen) show a dense concentration 9 (> 10 M ) of H2 in the nucleus. K99 describe NGC 5394 as `post-ocular' in that it resembles the `eye-shaped' inter- acting galaxies IC 2163 and NGC 2535 but is at a slightly later stage where `the eye-shaped oval has evolved into inner spiral arms and most of the gas has fallen into the central region, where it fuels a starburst'. K99 reported some evi- dence for a low-velocity outflow from asymmetry of the Hα line (a possible blueshifted component) and small (∼ 10 km s−1) distortions in the Hα velocity map (NE of the nucleus); unconfirmed but will be investigated here. Figure 1. Gran Telescopio Canarias public gallery im- age of Arp 84, NGC 5394 (north) 5395 (south). From Kaufman et al. (2002), hereafter K02, performed further www.gtc.iac.es/multimedia/media/GTC ARP84.jpg CO observations and comparison with several other pass- bands. They estimate NGC 5394 is inclined 15◦ ± 2 to the sky plane, discuss evidence (e.g. 60µm flux) the Hα flux is object' with a 8µm flux of 2:626 ± 0:015 mJy, ∼ 1% that reduced by as much as an order of magnitude by internal of the whole galaxy (279:8 ± 3:8 mJy), and in the mid-IR dust extinction, and on this basis estimate the total (dust- it is also detected in [NeII], [NeIII], [SIII] and polycyclic −1 corrected) star-formation rate (SFR) as 6±2 M yr (with aromatic hydrocarbons (PAHs). no evidence of an active galactic nuclei - AGN - contribu- Note that the SFRs estimated from observable quanti- tion). The star-formation closely traces the HI and H2 dis- ties will generally depend on the assumed initial mass func- tributions { all three are concentrated in the central kpc, tion (IMF), e.g. the SFR from K02 assumed a Salpeter IMF, but also the disk is `lopsided, with more HI, CO, and Hα whereas equivalent SFR estimates based on the Chabrier or emission coming from the western or southwestern side', and Kroupa IMFs, with fewer low-mass stars, will be 40{45% there was speculation that this resulted from a west-vs-east lower. Lehmer et al. (2010) estimated a (Kroupa IMF) SFR −1 time lag in the formation of disk arms and subsequent in- of 9.9 M yr for the whole Arp 84 system, simply from the 11 −1 fall of gas to the nucleus. They estimate a molecular/atomic infra-red luminosity of 10 M yr , and found its X-ray ratio M(H2)=M(HI) = 2:5{2.7 (even with a lower CO{H2 luminosity from Chandra data consistent with the typical conversion factor applied for starbursts), very high com- Lx − SFR relation of luminous infra-red galaxies (LIRGs), pared to 0.86 for NGC 5395, which contains much more HI meaning it can be accounted for by X-ray binaries without 10 (> 10 M ), and the average ratio of 0.28 for spiral galaxies requiring AGN. Howell et al. (2010) estimated a higher SFR −1 (Casoli et al. 1998). of 20.99 M yr by including the unobscured component Puerari, Valdez-Guti´errez& Hern´andez-L´opez (2005) traced by far-UV luminosity (GALEX) as well as the infra- studied the morphology of both galaxies in the near-infrared red luminosity and modelling with the Salpeter IMF. (JHK), and fit (in the K-band) a bulge profile with reff = Lanz et al. (2013) combined fluxes in many bands from 3:12 arcsec plus a disk of scale length h = 6:35 arcsec for FUV (GALEX) to FIR (Herschel) for a sample of interact- NGC 5394, which is `more compact than normal galaxies'. ing galaxies and fitted the SEDs with a program MAGPHYS, From the disturbed, Cartwheel-like structure of NGC 5395 which assumed a Chabrier IMF and gives a SFR averaged they conclude the system is undergoing a strong interaction, over the past 100 Myr. For NGC 5394 and 5395, they esti- −1 a `collision rather than a grazing encounter'. Kaneko et al. mate SFRs of 1.54 and 3.69 M yr , and found some differ- (2013) again study the system in CO and depict the contrast ence in their SEDs, with NGC 5394 having relatively more between the compact, nucleated, H2-rich NGC 5394 and the 12-60µm flux and higher dust temperatures (23.2K in the ring-like and HI-rich 5395. cold component) in comparison to 5395 (with 19.7 K) and Smith et al. (2007) observed the system from 3.6 to the non-interacting control sample (about 19K). This is con- 24µm in a Spitzer survey of Arp-catalog galaxies: most emis- sistent with NGC 5394 experiencing a strong (`stage 4'), sion from NGC 5394 is from the compact nucleus and the and late-stage interaction, with star-formation more cen- Spitzer images also show the prominent bright spot (Hα trally concentrated than in the more massive companion.
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